ioctl performs a variety of control functions on devices and STREAMS.
For non-STREAMS files, the functions performed by this call are devicespecific
control functions. request and an optional third argument with
varying type are passed to the file designated by fildes and are
interpreted by the device driver.
For STREAMS files, specific functions are performed by the ioctl call as
described in streamio(7). Argument types and other request-specific
information found in streamio(7) take precedence over information found
in this manual page.
fildes is an open file descriptor that refers to a device. request
selects the control function to be performed and depends on the device
being addressed. Many devices do not require that the device be opened
in a writable mode in order to successfully perform ioctls that change
the device parameters, or even the contents of the media for devices with
permanent storage. arg represents a third argument that has additional
information that is needed by this specific device to perform the
requested function. The data type of arg depends upon the particular
control request, but it is generally either an int or a pointer to a
device-specific data structure.
In addition to device-specific and STREAMS functions, generic functions
are provided by more than one device driver, for example, the general
terminal interface [see termio(7)].
The following operations are generally available on all descriptors [see
streamio(7)]:
FIONBIO Enables or disables non-blocking I/O on fildes. arg is a
pointer to an int whose value should be 1 to enable nonblocking
I/O or 0 to disable non-blocking I/O. This
operation is not currently supported on old-style (nonSTREAMS)
pipes.
FIONREAD Returns the number of bytes of data currently available to
be read on fildes. arg is a pointer to a size_t into
which the byte-count will be stored.
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ioctl fails for any type of file if one or more of the following are
true:
EACCES The type of access requested on the file designated by
fildes is denied.
EBADF fildes is not a valid open file descriptor.
ENOTTY fildes is not associated with a device driver that accepts
control functions.
EINTR A signal was caught during the ioctl system call.
ioctl also fails if the device driver detects an error. In this case,
the error is passed through ioctl without change to the caller. A
particular driver might not have all of the following error cases. Under
the following conditions, requests to device drivers may fail and set
errno to:
EFAULT request requires a data transfer to or from a buffer
pointed to by arg, but some part of the buffer is outside
the process's allocated space.
EINVAL request or arg is not valid for this device.
EIO Some physical I/O error has occurred.
ENXIO The request and arg are valid for this device driver, but
the service requested can not be performed on this
particular subdevice.
ENOLINK fildes is on a remote machine and the link to that machine
is no longer active.
ETIMEDOUT fildes is on a remote machine which is not available [see
intro(2)].
STREAMS errors are described in streamio(7).

Upon successful completion, the value returned depends upon the device
control function, but must be a non-negative integer. Otherwise, a value
of -1 is returned and errno is set to indicate the error.
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